A number of procedures are known for producing low molecular weight acrylic polymers. Achievement of the desired molecular weight and distribution is influenced by the type of process control applied (discontinuous, semi-continuous, continuous operation), by the reaction conditions (for example reaction temperature) and the materials system selected (solvent, initiator, chain-transfer agent). Thus, the solvent, initiator type and concentration and chain-transfer agent may purposefully be selected to achieve a desired molecular weight and distribution.
Discontinuous and semicontinuous methods in particular do not always proceed without complication, since undesirable proportions of homopolymer or copolymer having an undesirable composition or microstructure may be formed.
Semicontinuous production of (meth)acrylic polymers in the presence of polyester oligomers is known to the person skilled in the art. The polymer may be produced by free-radical polymerisation carried out according to conventional processes, wherein at least part and preferably at least half of the monomer mixture to be used for synthesis of the (meth)acrylic polymer is added to the initial polyester resin at a given reaction temperature. The polymerisation reaction may be initiated using known free-radical initiators. When polymerising, the monomers may also be added separately or with a time delay. The polyester resin is preferably initially introduced in the form of a solution in an organic solvent or in solvents conventional to the lacquer industry.
In general, processes of this type are carried out using an organic solvent. Furthermore, semi- and discontinuous modes of operation permit only limited obtainment of chemically uniform copolymers with the desired microstructure. Moreover, the narrow molecular weight distribution required for certain applications may be achieved only with difficulty.
EP-A-0 206 072 describes a semicontinuously operated process for the production of low molecular weight acrylic polymers. In general, a polymer material acting as solvent is initially introduced and heated to the reaction temperature, the vinyl monomer component and the free-radical initiator then being added in continuous manner. After addition of the components, the reaction continues until it is complete, the reaction product being isolated only thereafter. The vinyl monomer components may for example comprise conventional (meth)acrylates, styrene derivatives or glycidyl (meth)acrylates. The polymer material acting as solvent has a molecular weight of at least 200, preferably at least 1000 g/mol. It preferably contains functional groups which are substantially unreactive with the functionalities of the vinyl monomer component or of the polymer product. The polymer material may consist, for example, of polyester polyols. Although carrying out polymerisation with a combination of a high reaction temperature and a high feed rate does indeed result in a low molecular weight, non-gelled reaction product, a uniform product may be achieved only by taking elaborate measures to establish the reaction conditions. Furthermore, unreacted vinyl monomer components may be present in the reaction mixture, for example in an amount of up to 10 wt. %, relative to the reaction mixture.
According to U.S. Pat. Nos. 4,414,370, 4,529,787 and U.S. Pat. No. 4,546,160, low molecular weight vinyl polymers with narrow molecular weight distribution are produced by adding a vinyl monomer mixture continuously at a high reaction temperature and specified flow rates to a molten mixture of unreacted vinyl monomer and the vinyl polymer obtained therefrom. In addition to formation of the reaction product, this allows a certain amount of reaction mixture to be retained in the reaction zone. The reaction product is discharged from the reaction zone at the same rate as the monomer components are fed thereto. Unreacted monomers may be reused by being returned to the reaction system. The viscosity of the product may be reduced and the reaction parameters may optionally be controlled by the use of a solvent, for example higher aromatic alcohols, glycol ethers and glycol esters.